The present invention relates to improvements in a camera having a photoelectric focus detecting device.
The conventional photoelectric focus detecting device of a camera has detected electric signals of a focused condition detecting circuit, of which levels are decreased or increased according to the focused condition, thereby to judge that it is under the focused condition when the aforementioned electric signals are lower or higher than a preset level for judgement.
In a single lens reflex camera, on the other hand, the system, in which the focus detection of the so-called "TTL" type is adopted, has been conceived by arranging the light receiving unit of the photoelectric focus detecting device in an optical path at the back of an objective lens. In the single lens reflex camera, generally speaking, since an object to be photographed is usually observed under the condition that an aperture of an iris diaphragm is in a wide-open state, the focus detecting operations are often performed with the wide-open iris diaphragm, too. In the conventional focus detecting device of that type, therefore, the range of depth of focus under the wide-open iris condition has been used as a reference level or range for focus judgement after they had been converted into the judgement level or range.
Here, the case of a camera of interchangeable lens type will be discussed. Since the interchangeable lenses are different in their wide-open F-number, focal length, lens construction and the like, they have accordingly different ranges of depth of focus when the iris is widely open. As a result, in case the level for judgement at the side of the focus detecting device is preset such that the level of a certain lens is used as a reference, a difference between the focused condition displayed in accordance with the judgement level and the actual focused condition on a film plane may be invited depending upon the kind of lenses interchanged. For instance, in case a lens having a small wide-open F-number is mounted on a camera having the aforementioned judgement level preset on the basis of such a range of depth of focus (e.g., its high value) with the wide-open iris as belongs to the lens having a relatively large wide-open F-number, the display of "It is in focus." is made by the focused condition display based on the judgement level, even if the actual focused condition on the film plane still fails to form a clear image, because there is a difference in the range of depth of focus. This results in formation of an image out of focus. In the contrary case, moreover, the display based on the judgement level indicates "It is still out of focus.", although a clear image has already been formed on the film plane. This invites the result that a photographer has to continue the focusing operations which might otherwise be unnecessary.
The results thus far described are also invited by the difference in the focal length and in the lens construction and are accordingly detrimental to the device of that type.
Such discrepancy between the focused condition displayed based on the judgement level and the actual focused condition on the film plane, which is caused by the difference in the range of depth of focus, takes place in the cases other than that of the lens interchange, too. More specifically, the first one is the case, in which the setting of the judgement level is performed on the basis of the range of depth of focus with the iris being widely open whereas the actual photography is accomplished with the iris being partially open. In the second case, the focal length is changed for a lens having a variable focal length such as a zoom lens.
The former case often takes place with a single lens reflex camera adopting the focus detecting system of "TTL" type. More specifically, in the case of the single lens reflex camera, the observation of an object to be photographed has to be performed under the condition having the iris widely opened, so that the focus detecting operation cannot be performed under any other condition than that with the widely open iris, as has been discussed before. In this instance, moreover, since the focus detection has to be effected even with the widely open iris, the setting of the judgement level is often based on the range of focus under the same condition. Since, however, the normal photography is usually accomplished with an aperture or iris diaphragm in a range of F-number of 4 to 8 having a relatively high range of depth of focus, this case is also accompanied with one of the aforementioned disadvantages that the display based on the judgement level indicates "It is still out of focus." although a clear image has already been formed on the film plane.
In the latter case, on the other hand, a problem is raised by a zoom lens of the type, in which the minimum F-number and accordingly the range of depth of focus is varied upon the zooming operation in accordance with the change in the focal length. Similar disadvantage to those of the foregoing two cases is also invited.
In case, on the other hand, the judgement level is set by the use of the range of depth of field in place of the range of depth of focus, similar disadvantage to the aforementioned one coming from the difference in the range of depth of focus is also invited.